Ambient air quality affects the health of people breathing the ambient air. The lower the air quality, the greater the risk for health-related problems induced by the ambient air. Conventional particulate matter monitoring devices measure the mass concentration of particulate matter within ambient air, gases, or other fluids to determine quality of the ambient air or gaseous fluid. A conventional particulate matter monitoring device can provide a warning to a user when the device detects a relatively low air quality (e.g., a relatively large particulate mass concentration within the air) or a decrease in the ambient air quality based upon an increase in particulate mass concentration measured over a specific time period.
Several sensing techniques provide continuous direct monitoring of airborne particulate mass concentration. The sensing techniques used for particulate monitoring include, for example, mass sensing methods such as beta radiation attenuation and optical sensing methods such as light scattering photometry or nephelometry.
Beta radiation attenuation devices typically include a mass sensing stage and a particle collection stage. The mass sensing stage includes a beta particle radiation source, typically carbon-14 or krypton-85, and a beta radiation detector, typically a Geiger-Muller detector, plastic scintillator, a proportional counter, or an ionization chamber. The particle collection stage includes a filter oriented between the beta radiation source and the beta radiation detector. Beta radiation attenuation devices exhibit a substantially exponential attenuation characteristic as a function of the mass per unit area collected by the filter between the radiation source and the radiation detector. For example, during operation, ambient air (or another gaseous compound) flows through the filter and the filter collects particulate matter present within the ambient air over time. As the amount of particulate matter collected by the filter increases, the particulate matter attenuates the beta radiation emitted from the radiation source (i.e., the beta radiation detector senses less radiation from the beta source) as detected by the detector. Because the attenuation of the beta radiation detected by the beta detector is related to the mass of the particulate matter collected by the filter (e.g., and does not substantially depend upon the type or compound of material collected by the filter), a beta radiation signal produced by the beta radiation attenuation device indicates a mass concentration of particulate matter within an air sample.
Beta radiation attenuation devices, and other mass sensing devices that utilize a filter to collect particulate matter within an air sample, sometimes utilize temperature and humidity conditioning elements to remove liquid water from the air sample. Typically, air samples include varying amounts of water and water vapor. If the filter collects water over time, the water adds to the attenuation of the beta radiation from the beta radiation source to the beta detector. The water that collects in the filter, therefore, can affect the accuracy of the device's detection of mass of particulate matter within the air sample. In conventional beta radiation attenuation devices, prior to the air sample reaching the filter, the device heats the air sample to ensure that the water in the air and on the particulate matter evaporate from the air sample. By keeping the water within the air sample as a vapor instead of condensation, the temperature and humidity conditioning elements can increase the particulate matter detection accuracy of the device by reducing or eliminating collection of water in the filter.
Another type of conventional particulate mass sensing device is a light scattering device known as a nephelometer. Light scattering photometry devices, such as nephelometers, measure the irradiance of light scattered by particles passing through a sensing volume. Typical light scattering photometry devices include a light source that creates an illuminating light beam and detection optics or sensors that measure the strength of the scattered light. During operation, ambient air or another gas flows through a sensing volume defined by an intersection of the illuminating beam and the field of view of the sensing optics. As the air flows through the sensing volume, the light source illuminates particles present within the sensing volume, and the optics and associated photosensitive measuring circuitry detect the light scattered by the particles. For an ambient air sample having a fixed size distribution of particles with invariant density and index of refraction, the intensity of light scattered by the particles within the air sample is directly proportional to the mass concentration of the particles within the air sample. Light scattering photometry or nephelometry devices, therefore, allow real-time (i.e., substantially instant) measurement of particulate mass concentration of ambient air.